Technical Field
The present invention relates to the technical field of power transmission and distribution, in particular to a tripolar High Voltage Direct Current (HVDC) transmission system for converting an existing AC transmission line to a DC transmission line.
Related Art
Along with rapid development of economy in China, a power requirement conflict is urgent day by day. On one aspect, increase of a power load exceeds a transmission capacity of an original AC transmission line, and a new power corridor transmission is hard to get due to limitation of a land resource; on the other aspect, traditional AC transmission is limited by insulation and AC power grid characteristics, a line transmission power hardly achieves the maximum thermal power that the conductor can bear, and therefore, a new technique is urgently required to develop the power transmission potential of the existing lines.
In order to make full use of the existing AC line transmission corridor, besides the direct increase of the line operating voltage level, following techniques can be adopted to improve the line transmission capacity: a fixed series parallel compensation technique, a flexible AC transmission technique, a compact power transmission technique, a line dynamic capacity increasing technique, a technique of improving a working allowable temperature of a conductor, a novel conductor power transmission technique and a technique of converting an AC line into a DC line.
Transmission power of an AC line is mostly limited by characteristics of an AC power grid, and it is usually far lower than the thermal maximum of the conductors. Compared with AC transmission, a DC transmission line current can achieve the thermal maximum of the conductor, and the DC line is lower in construction cost and small in loss. When the saved cost of the DC line is enough to offset the increased cost of building a new converter station, the economic benefits of DC transmission are more obvious. In addition, the DC transmission has the advantages of power control, no increase of a system short circuit current, without skin effect of the line, higher insulation utilization rate of the line or cables, fast regulating speed, reliable operation and capability of realizing asynchronous connecting of AC systems.
In 2004, American scholar Barthold L O provides a tripolar DC transmission system based on a conventional LCC-HVDC as shown in FIG. 1 in U.S. Pat. No. 6,714,427B1, a principle thereof is to adopt a current modulation technique to convert an AC transmission line into a DC transmission line. Compared with a bipolar and a monopolar DC transmission conversion solution, a tripolar DC transmission solution can use all original AC three-phase lines, and has advantages on aspects of improving power transmission capacity, economic cost and reliability. At present, conversion of AC lines to DC lines by the tripolar DC transmission technique is still in principle research stage and no project instances exist. The LCC-HVDC system adopts a semicontrolled thyristor, the tripolar DC transmission solution in FIG. 1 has following defects: (1) if an inverter AC system has a ground fault, it is possible that three DC poles are failed in commutation at the same time, resulting the DC transmission power is interrupted, And the security and stability of the receiving end system is threatened. (2) during the operation of the tripolar DC system, the current value of each pole, the voltage polarity and current direction of the pole 3 are required to be fast changed according to certain period, which certainly will generate disturbance to the AC system at two ends. (3) since each line is configured with a complete pole, more converter transformers, AC filters, reactive compensation devices, converters and corresponding auxiliary devices are required, which resulting in a higher reconstruction investment, and higher occupied space of the new converter station, and it is especially unfavorable for implementation of a project of converting AC lines to DC lines in large cities with strict limit on the occupied space of the converter station.
In recent years, the VSC-HVDC technique, which adopts the full controlled power electronic device IGBT, is developing rapidly. Compared with the traditional LCC-HVDC transmission technique, the VSC-HVDC technique has the advantages of no need of commutation voltage, support by power grid, capability of providing power to a passive network, no commutation failure, capability of realizing independent control of active power and reactive power and no need of AC filter and reactive compensation devices. Differentiated from a structure of a converter, the VSC-HVDC system mainly comprises two level converters, a tri-level converter and a modular multilevel converter (MMC) structure, wherein the MMC-based VSC-HVDC system not only has the VSC-HVDC general advantages, but also has the characteristics of low switching frequency, small loss, easy expansion, high quality of output waveforms, low manufacturing difficulty etc, Therefore, MMC-based VSC-HVDC has broad application prospects.
In order to solve the problems of the conventional LCC-HVDC based tripolar DC system, reduce the area of the new converter station and improve the economy of the solution of converting AC lines to DC lines, the present patent provides a tripolar VSC-HVDC system based on modular multilevel converter (MMC), which can well meet the requirements of improving transmission capacity in the urban power supply system using the solution of conversion AC lines to DC lines.